The number of violent crimes in the U.S. increased 3.9 percent in 2015 from a year earlier amid a jump in murders, according to annual crime statistics released by the FBI.

Murders and non-negligent manslaughter increased 10.8 percent last year compared to estimates from 2014, while rapes and aggravated assaults increased 6.3 percent and 4.6 percent, respectively, according to the report published Monday. By total numbers, there were 15,696 murders and non-negligent manslaughter offenses in 2015.

The findings come just hours before Hillary Clinton and Donald Trump square off in their first presidential debate, with security issues expected to be a major topic of discussion. Five people were killed by a gunman in a mall in Washington state on Friday, and violence between police and citizens has become a frequent focus of this presidential election season.

"We still have so much work to do," Attorney General Loretta Lynch said about the report. "It is important to remember that while crime did increase overall last year, 2015 still represented the third-lowest year for violent crime in the past two decades."

Lynch suggested some of the headline numbers masked accomplishments by law enforcement. The property crime rate, for example, declined 3.4 percent.

Overall Trend

"The report also reminds us of the progress that we are making." Lynch said. "It shows that in many communities, crime has remained stable or even decreased from the historic lows reported in 2014."

Overall, the violent crime rate has steadily decreased and has been nearly cut in half over the last two decades, according to the report. In 1996, for example, the violent crime rate was 636.6 per 100,000 people. By 2015, it had fallen to 372.6.

Among major U.S., cities, there were 162 murders and non-negligent manslaughter offenses in Washington, D.C., last year compared to 478 in Chicago, 352 in New York and 282 in Los Angeles, according to the report.